Biomethylation of heavy metals has been the subject of great interest for more than one hundred years and has become accepted as a common, but important, chemical process occurring in the environment for many elements, including tin, mercury, iodine, bromine, and sulfur [1]. More recently, bio-transformation of antimony and arsenic compounds to volatile toxic species has been linked to sudden infant death syndrome (SIDS) [2-3].
Although almost all emphasis has been placed on biomethylation, some recent research has shown that a methylchromium bond has been formed during the photolysis of tert-butoxy radicals and chromium (II) in aqueous solution [4]. In the presence of acetate ion or acetic acid in aqueous solution, Hg2+ gives rise to methylmercury following photolysis in sunlight (UV light) [5]. By using X-ray irradiation, Landner obtained mercury-resistant strains of Neurospora, which produced more methylmercury than the parent strain [6]. In monocultures of some phototrophic bacterial cultures amended with tellurate or elemental Te (powdered metal), dimethyltelluride was detected after 7 days of growth in the light [7]. In the case of selenium, use of a TiO2 photocatalyst and UV irradiation, permitted removal of selenate ions from an aqueous solution, presumably by formation of volatile SeH2 [8]. Synthetic seawater, spiked with organo-selenium compounds and exposed to radiation from “sunlight’, produced methylated selenium, which was not the case with spikes of inorganic selenium [9].
Bio-methylation of heavy metals has been the subject of great interest for more than one hundred years. Initial investigations centered on the identification of a garlic odor, which was often present in damp rooms decorated with green wallpaper pigmented with arsenic compounds. When fungal growth was favored, illness and fatalities often resulted for those who slept in such rooms. In 1901, Gosio identified a volatile methylated arsenic compound having a garlic odor, and suggested that diethylarsine was released from moulds growing in the presence of inorganic arsenic [12]. The fungus, originally termed Penicillium brevicaule, latter called Scopulariopsis brevicaula (S. Brevicaulis), produced the garlic smelling gas, which had been the cause of a number of intoxication incidents. However, a re-examination of the issue by Challenger and colleagues in 1933 conclusively demonstrated that Gosio Gas was, in fact, trimethylarsine [13-14]. Considering that selenium may have also been present as a contaminant in the arsenic pigments, work in his laboratory soon demonstrated that methylation of selenium to dimethylselenide was also possible [15]. Since then, biomethylation has become accepted as a common, but important, chemical process occurring in the environment for many elements, e.g., for tin, mercury, iodine, bromine, and sulfur [16]. More recently, bio-transformation of antimony and arsenic compounds to volatile toxic species has been linked to sudden infant death syndrome (SIDS) [17-18]. In the 1950's, the devastating cases of “Minamata disease” (methylmercury poisoning) in Japan accelerated investigations into this phenomenon.
Selenium has been identified as an essential trace element; excessively low or high dietary intake results in toxicity and thus constitutes a threat to personal health [19, 20]. Obtaining sufficient selenium in the diet may protect against cardiovascular disease [19], viral infections (including influenza [21] and HIV [22, 23]), rheumatoid arthritis [24], liver disease [25], and serve to detoxify heavy metals as well as prevent some forms of cancer [26-27]. Selenium-binding enzymes, glutathione peroxidases (GPx), are responsible for eliminating such harmful oxidants as hydrogen peroxide and lipid peroxides. A deficiency of active Se-bound GPx appears to play a crucial role in the pathology of many conditions associated with selenium deficiency [20, 23, 28]. At higher selenium concentrations, the metal and its salt often inhibit biological activity. Selenite is much more toxic than selenate, both in vivo and in vitro [29-30], however, dimethylselenide is 500- to 700- fold less toxic to rats than aqueous selenite and selenate [31-32]. As one of several detoxification processes, it has been known for over a century that bacteria are capable of reducing selenium salts to elemental selenium [33]. In the presence of stomach acid, selenite is converted to selenious acid and is further converted to inactive, elemental selenium if vitamin C is ingested simultaneously [34]. Many suboxic sediments and soils contain an Fe(II, III) oxide, which reduces selenium from an oxidation state of +VI to 0 in the natural environment [35]. Bacteria [36-41], plants [42-43], decaying plant detritis [44], marine algae and plankton [46-47] as well as animals [48] have all been shown to be capable of methylating selenium from the selenate (VI) and selenite (IV) oxidation states, even from the elemental selenium state. Rats fed selenate or selenite exhale dimethylselenide [48]. A garlic odor present in the breath of workmen engaged in the extraction of selenium from electrolytic copper “slimes” has also been detected by Dudley [49]; the exhaled product is almost certainly dimethylselenide [15]. Consequently, bio-methylation of selenium in soils, sediments, plants, fresh water systems and the marine environment into DMSe is considered to be a major source of atmospheric selenium [50-51]. Losses of selenium from soils by bio-methylation may, in some cases, give rise to an insufficient supply of selenium to animals [52]. The reduction of Se oxyanions to Se0, followed by further reduction and methylation to a volatile methylated form is commonly regarded as the most important detoxification route in biological systems. Such processes have also been widely applied, despite numerous unpleasant byproducts which may be produced, for reducing Se contamination in wastewater arising from agricultural drainage systems, power plants, oil refineries and electronic industries [53-57].
Although almost all emphasis has been placed on bio-methylation, some recent research has shown that a methylchromium bond has been formed during the photolysis of tert-butoxy radicals and chromium (II) in aqueous solution [58]. In the presence of acetate ion or acetic acid in aqueous solution, Hg2+ gives rise to methylmercury following photolysis in sunlight (UV light) [59]. By using X-ray irradiation, Landner obtained mercury-resistant strains of Neurospora, which produced more methylmercury than the parent strain [60]. In monocultures of some phototrophic bacterial cultures amended with tellurate or elemental Te (powdered metal), dimethyltelluride was detected after 7 days of growth in the light [61]. In the case of selenium, use of a TiO2 photocatalyst and UV irradiation, permitted removal of selenate ions from an aqueous solution, presumably by formation of volatile SeH2 [62]. Synthetic seawater, spiked with organo-selenium compounds and exposed to radiation from “sunlight’, produced methylated selenium, which was not the case with spikes of inorganic selenium [63].
These studies clearly indicate that photolysis may play a significant role in the transformation of heavy metals in the environment. However, there have been no reports on the role of direct photochemical alkylation of inorganic selenium. The unique photoelectric and semiconductor properties of this element have been widely utilized in photocell devices and in xerography, solar batteries, specialty transformers and rectifiers, all serving to release it into the environment where some fraction is transformed into organoselenium compounds by biological systems. Despite much progress in understanding bio-methylation as a link between inorganic and organic selenium under natural conditions, this goal remains elusive. At the molecular level, reduction and bio-methylation of selenium with or without involvement of light is not well understood. Photosynthesis is a most important process in nature by which green plants, alga, and photosynthetic bacteria use energy from sunlight to stimulate chemical reactions in plants, it may play a role in reduction, bio-methylation and mobilization of inorganic selenium, but this remains to be clarified.
In 1879, Mond discovered that carbon monoxide, passed over finely divided nickel metal, formed gaseous nickel tetracarbonyl [Ni(CO)4]. This is a readily reversible reaction in that the carbonyl can be decomposed to yield nickel metal and carbon monoxide at 180° C. [102]. The resulting Mond Process became one of the truly elegant metallurgical procedures and the discovery was also a notable step in the history of organometallic chemistry, which led to Reppe catalysts (using nickel carbonyl for synthesis). Nickel catalyzes the gasification of biomass and is also operative in biological systems [103-108] catalyzing biochemical reactions. The discovery of new synthetic methods for production of nickel carbonyl and its cluster species is of interest not only for application to new commercial materials, but is also of theoretical significance, for it enriches our information of metal cluster bonding, permitting further insight into an understanding of chemical reactions under various conditions.
Methods for the production of Ni(CO)4 have been documented in 55 patents since 1955 [109] most of them dealing with dry contact methods for the reduction of ores, oxides, or salts for preparing the highly active metal, followed by reaction with carbon monoxide. The discovery of new and easier preparative procedures for organometallic compounds has always been regarded as a great rebirth or expansion of classical organometallic chemistry [110].
However, in many circumstances, bio-detoxification is unavailable or insufficient to remove contaminants such as metal ions from aqueous environments. Accordingly, there is a need for others means to reduce, alkylate or carbonylate ions of noble and transition metals in aqueous environments.